This invention relates to method and apparatus for storing hydrogen in and recovering hydrogen from a hydride material.
As a result of recent shortages in hydrocarbon fuels and the recognition that the supply of such fuels will ultimately be exhausted, there has been an increased interest in finding and developing alternative fuels. One alternative fuel whose potential has long been recognized but, as yet, has not been realized is hydrogen. The attractiveness of hydrogen as a fuel lies in the fact that it is one of the most abundant of all elements, that conventional internal combustion engines can be readily adapted to operate on hydrogen and in such operation, unlike gasoline, a large percentage of the hydrogen is converted to power the engines, and that the burning of hydrogen in such engines can be made to be relatively pollution free. See, for example, copending application, Ser. No. 554,533, filed Mar. 3, 1975, now U.S. Pat. No. 3,983,882. Of course, the potential of hydrogen as a fuel is not limited to internal combustion engines but also extends to industrial uses, use in fuel cells and in home and mobile home heaters, and to any situation where natural gas, propane gas, etc., is presently used.
One of the problems which has thus far prevented the widespread use of hydrogen as a fuel has been the difficulty in efficiently and safely storing the hydrogen. Storing hydrogen as a liquid is costly since it requires considerable power to liquify the hydrogen and transfer of the liquid from one container to another results in a loss to the atmosphere of much of the hydrogen. Also, containers for the liquid hydrogen must be extremely well insulated and sturdy to reduce the loss of hydrogen due to vaporization or boiling. Storing hydrogen as a gas requires extremely heavy and bulky containers and is impractical for most presently contemplated consumer uses.
The use of hydride material (hereinafter defined to mean any metals, metal compounds or other materials capable of absorbing and holding hydrogen) appears to be an attractive approach to the storage of hydrogen for consumer purposes. Exemplary hydride material includes iron titanium, misch-metal nickel, and columbium. Storage of hydrogen in the hydride material (sometimes referred to as hydrating the material) typically involves lowering the temperature of the hydride material and then applying hydrogen gas under pressure to the material. After the hydride material absorbs the hydrogen, the material is sealed in a container under pressure to maintain the material in the hydrated state until the hydrogen is needed at a subsequent time. Recovery or withdrawal of the hydrogen involves a process substantially opposite that used for storing the hydrogen, i.e., heating the hydride material and releasing some of the pressure of the container in which the hydride material is maintained.
Structure heretofore used for holding the hydride material and storing the hydrogen has typically included a storage container having a plurality of conduits running through the container. The hydride material is placed in the container and a heat exchange medium passed through the conduits either to cool the hydride material, when storing the hydrogen, or to heat the hydride material, when releasing the hydrogen. One of the problems with this type of structure, because of the geometry of the conduits relative to the hydride material, is that some of the hydride material is disposed in locations too far from the conduits to readily absorb or release hydrogen. That is, because the heat transfer path between the heat exchange medium and some hydride material is long, either the hydrogen will not be absorbed or released from such material, or the time required for such absorption or release is longer than desired. Of course, increasing the number of conduits helps but this also reduces the space available in the container for holding the hydride material and increases the weight and cost of construction of the container.
Another problem with the prior art containers arises from the increase in temperature (during hydrogen storage) or a decrease in temperature (during hydrogen release) as the heat exchange medium passes through the container. Specifically, the rate of heat exchange decreases as the heat exchange medium moves through the container so that for the hydride material near the exit end of the container, the rate of absorption and release of hydrogen will be reduced and may be too low for full utilization of such hydride material.